where, k2a=0.405±0.055 M-1 min-1 at 55 °C (β3≈0.08; β4≈0.007) and the activation energy was E=112±17 kJ mol-1. The reaction mechanism appeared to involve interaction with the 1st hydrolysis product of U(IV)–UOH3+. The data is compared with a previous study of the nitric acid oxidation of U(IV) in 30% TBP. This reaction is autocatalytic due to the formation of nitrous acid during the reaction. The kinetics of the decomposition of HNO2 in 30% TBP (in the absence of U(IV)) have also been reported.
The rapid reduction of NpO22+ ions to NpO2+ by U(IV) ions in an aqueous nitric acid solution has been studied and, in many ways, is similar in character to the same reaction in HClO4. The major difference is that in HNO3 the reaction proceeds via two parallel routes. The first is via the hydrolysed UOH3+ ion, as in the reaction in HClO4 and the second is via the non-hydrolysed U4+ ion. These parallel routes lead to the observed order of reaction with respect to H+ ions being reduced from −1 in HClO4 to −0.7 in HNO3. After accounting for the reaction mechanism the rate equation is described by:
where k8 = 404 min−1, k19 = 275 M−1 min−1 and β = 0.009 M at 10 °C and μ = 2. The activation energy was 66.5 ± 4.9 kJ mol−1. Nitrate ions had no effect on the reaction rate but sulphamic acid increased the observed rate, probably through catalysis by sulphate ions arising from sulphamic acid reaction with HNO2 and hydrolysis.
The oxidation of U(IV) ions in the diluted solvent phase, 30% TBP/n-dodecane, has been investigated in the presence of plutonium ions, which can act as catalysts for U(IV) oxidation. The reaction was shown to follow the cycle below, with the first and third stages being rate determining.
U4+ + 2Pu4+ + 2H2O → UO22+ + 2Pu3++4H+
2Pu3+ + HNO3 + 2H+ → 2Pu4+ + HNO2 + H2O
Pu3+ + HNO2 + H+ → Pu4+ + NO + H2O
2NO + HNO3 + H2O ⇔ 3HNO2
The overall reaction stoichiometry is the same as for the oxidation of U(IV) by HNO3 in TBP:
The rate equations of both these rate limiting steps have been determined, with that for the U(IV)-Pu(IV) reaction (5) being given by the equation below, where k1=74.4±6 M-1.2 min-1 at 25.2 °C and the activation energy is 72±11 kJ mol-1 (in 0.5 M HNO3).
The rate of the second slow stage, the Pu(III)-HNO2 reaction, is given by the equation below, where the rate constant is k2=627±28 M-1 min-1 at 25.2 °C and the activation energy is 87.2±1.4 kJmol-1 (in 0.5 M HNO3).
Mechanistically, it was shown that the U(IV)-Pu(IV) reaction may proceed via the interaction of the hydrolysed actinide ions U(OH)22+ and PuOH3+ and the Pu(III)-HNO2 reaction was found to most probably involve oxidation of Pu(III) ions by nitrinium nitrate (NONO3) ions in its rate determining step.